The core of a nuclear reactor comprises a plurality of nuclear fuel bundle assemblies, each assembly consisting of a plurality of nuclear fuel rods. Each fuel rod comprises a circular cylindrical housing, i.e., cladding, which is hermetically sealed at both ends by respective end plugs. A plurality of nuclear fuel pellets are stacked in a vertical column inside the cladding to a height less than the length of the cladding, leaving a plenum space above the fuel column, which plenum space is filled with inert gas. A getter for removing contaminants from the inert interior atmosphere is conventionally installed inside the plenum.
One conventional type of fuel rod 14 (see FIG. 1) has a circular cylindrical housing 17, i.e., cladding, made of corrosion-resistant metal, e.g., zirconium alloy. The nuclear fuel is housed in cladding 17 in the form of a column of stacked pellets 16 made of fissionable and/or fertile material. Each fuel pellet is a circular cylinder having planar end faces disposed perpendicular to the cylinder axis. The pellets are stacked with end faces in abutment. A preferred fuel is uranium dioxide or a mixture comprising uranium dioxide and Gadolinia.
The cladding 17 is sealed at both ends by means of end plugs 18, only one of which is shown in FIG. 1. The end plugs are also made of zirconium alloy. Each end plug is joined to the cladding 17 by a circumferential weld generally indicated by numeral 22. Each end plug has a stud 19 which fits into a corresponding aperture in one of the tie plates to facilitate mounting of the fuel rod in the fuel bundle assembly.
During construction of the conventional fuel rod, a first end plug is inserted in an end of the cladding and welded circumferentially to form an airtight seal. The fuel pellets 16 are then inserted in the cladding to form a column, with the first pellet abutting the first end plug. When the fuel bundle assembly is installed upright in the reactor core, the fuel column has a height which is less than the height of the cladding, so that a void space or plenum 20 is provided at the upper end of the upright fuel rod.
In accordance with the construction of a known fuel rod, a standoff element 25 comprising a U-shaped standoff wire 25a sitting on a wafer-shaped base 25b is installed in plenum 20. The wafer-shaped base 25b sits on the top surface of the last pellet in the fuel column. Standoff element 25 supports a getter 23 at a predetermined height above the fuel column.
A conventional fuel rod further comprises a coiled compression spring 26 installed in plenum 20. Plenum spring 26 serves to maintain the position of the fuel pellets during handling and transportation of the fuel rods by biasing the fuel pellets toward the plugged end adjacent to the fuel column.
The plenum 20 is closed off by welding the second end plug 18 in the opposite end of cladding 17. This end plug is made of zirconium alloy. In contrast, the plenum spring 26 is stainless steel. Thus, if plenum spring 26 were in direct contact with end plug 18 during welding of the latter to cladding 17, the heat produced during welding could cause a chemical reaction between the stainless steel and the zirconium alloy. Such a chemical reaction can cause contamination of the end plug and possibly the weld.
To obviate this problem, an insert 24 made of Zircaloy is installed between end plug 18 and plenum spring 26. The conventional insert 24 is a tightly wound wire which does not act like a spring. Plenum spring 26 is provided at its end with a tightly wound wire portion 26a dimensioned to snugly receive the screwed-in insert 24. Alternatively, an insert of solid Zircaloy may be press-fit into the end of the plenum spring.
For final assembly of the fuel rod, plenum 20 must be filled with inert gas and hermetically sealed from the exterior of the rod. After the assembly comprising elements 23-26 is inserted in plenum 20, the end plug 18 is pressed against the insert 24 and fitted into the open end of cladding 17. Because this has the effect of compressing plenum spring 26, a downward axial force must be applied to hold end plug 18 in place. End plug 18 is then joined to cladding 17 by circumferential weld 22 to form a gastight seal.
End plug 18 has a radial pressurization hole 28b in fluid communication with a central axial bore 28a. The lower end of central axial bore 28a is in turn in fluid communication with plenum 20, while the radially outer end of pressurization hole 28b is in fluid communication with the exterior of the fuel rod. After end plug 18 has been welded to cladding 17, plenum 20 is evacuated and then back-filled with helium via pressurization hole 28b. The pressure of the helium is typically between 1 and 20 atm. The pressurization hole 28b is then spot welded to seal plenum 20.
Both the circumferential weld 22 and the spot weld in pressurization hole 28b must be of high integrity and must satisfy predetermined quality assurance criteria. The application of an ultrasonic probe having dual transducers permits the simultaneous, nondestructive evaluation of both the high-pressure spot weld and the circumferential weld for weld integrity characteristics.
The conventional welding method involves tungsten inert gas ("TIG") welding of pressurization hole 28b while end plug 18 is in a sealed weld box pressurized with helium. The weld process involves striking a high-current arc from a tungsten electrode to the pressurization hole 28b for 0.5 sec. The heat from the weld causes a rise in pressure of both the gas inside and the gas outside the welded pressurization hole. This system produces a spot weld 30 with a convex surface as shown in FIG. 2A.
During ultrasonic inspection of the integrity of spot weld 30, the ultrasonic transducer (not shown) is positioned to transmit ultrasonic energy into the weld along the axis of the pressurization hole 28b. The curvature at the outer edges of the convex surface of spot weld 30 causes much of the ultrasonic energy to reflect away from the transducer during inspection, thereby causing problems with the ultrasonic inspection system. Specifically, the points of minimum thickness cannot be accurately detected.